fix(outputs.postgresql): report a missing table instead of panicking

[arpitjain099]

Fixes #19501.

Render builds allColumns with tcs.Concat(table.Columns) without checking table, and NewTable returns nil whenever the table name is empty. A nil table therefore segfaults inside the template render and takes the agent down, which is the crash in the report:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x20 pc=0x6b5f965]
sqltemplate.(*Template).Render(...)
        plugins/outputs/postgresql/sqltemplate/template.go:397

I reproduced that exact frame from a unit test by handing Render the nil that NewTable("", "", nil) returns, so it is reachable from any caller that ends up with an empty table name, not only through the reported configuration.

Render now returns cannot render template without a table instead. The write then fails the way any other schema error does, gets logged by the table manager, and the agent keeps running.

To be clear about the scope: this stops the crash and turns it into a reportable error, but it does not explain how the table state reaches update with an empty name in the reporter's setup. That is worth chasing separately, and this change makes it visible in the logs instead of a stack trace.

TestRenderWithoutTable covers both directions, the nil table erroring and an ordinary table still rendering. It panics on master rather than failing.

go test ./plugins/outputs/postgresql/... passes.

[telegraf-tiger]

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[srebhan]

Thanks for your contribution @arpitjain099! Can you please restore the PR description template, especially the AI section, as we cannot review or merge your PR otherwise...